Process of reacetylating cellulose acetate



Patented May 8, l 928.

UNITED [STATES PATENT OFFICE.

I HA NS '1. CLARKE AND CARL J. MALM, OF ROCHESTER, NEW YORK, ASSIGNORS TO EASTMAN KODAK COMPANY; OF ROCHESTER, NEW YORK, A CORPORATION 01' NEW YORK.

PROCESS OF REAGETYLATING CELLULOSE ACETATE.

N'o Drawing.

This invention relates to processesof reacetylating cellulose acetate. One ob e ct of the invention'is to provide a simple, lnexpensive process' for partly reacetylating cel- 6 lulose acetate from which acetyl groups have been split off, such as by hydrolysis. An other object is to provide a simple and easily controlled rocess for adjusting or restoring the solubihty of hydrolyzed cellulose ace- 10 tate in organic solvents, such as acetone. Another object is toprovide a process for restoring acetone-solubility in cellulose acetate which has been over-hydrolyzed to the point where such solubility has .been 11n- 15 paired. Other objects will hereinafter apear.

p Cellulose acetate has often been made by acetylating it as far as practical and thus obtaining chloroform-solubility. This product has then been hydrolyzed until a product is obtained which is soluble in acetone. Careful analysis has shown that the change from one solubility to the other is accomplished by a loss of acetyl roups during the hydrolysis. In other wor s, the acetonesolublecellulose acetate contains less combined acetic acid than the chloroform-soluble species from which it was derived.

The hydrolysis may take place while the cellulose acetate is in solution in acetic acid, as in U. S. Reissue Patent No. 12,637 Miles, April 23rd, 1907, orit may take place by the hydrolysis of the solid cellulose acetate in an acid solution, as described, for example, in British Patent No. 24,067, Newton,

' of 1906. The solid cellulose acetate during this treatment may conveniently be in the form of a spray-dried powder, as explained in U. S. Patent No. 1,516,225, Webb, No-

4 vember 18th, 1924'. A deacetylating action may also be brought about by a moderate saponification, as indicated in U. S. Patent No. 1,074,092, Mork, September 23rd, 1913,

and No. 1,061,771, Mork, May 13th, 1913.

Itis manifest that in all of these cases the action may be inadvertently carried too far.

In the case of the acid treatment, either while the cellulose acetate is dissolved in acetic acid, or While it is in the form of solid par- Application filed Januarym, 1927. Serial No. 160,766.

ticles, too long a treatment may greatly lessen, or even eliminate the solubility in acetone produced by the initial stages of the treatment. In the alkaline. saponification the process may be carried so far that a reverted cellulose may be obtained. As the purpose of these processes was to obtain certain properties by splitting off part of the acetyl groups, it is undesirable to correct any excessive removal of the groups by replacing all of them. And yet a retreatment in the ordinary acetic acid-acetic anhydridcatalyst bath is practically sure to do this, giving, for example, a chloroform-soluble product. It is quite impractical, and very expensive, to carry such a reacetylation to only a partial degree,-for example, to restore normal acetone-solubility to cellulose acetate which has been rendered sparingly soluble in that liquid. Substantially a full acetylation must be obtained and then a second and correct hydrolysis carried out. It is manifestly desirable that there be some process which will, with accurate control, restore just enough of the removed acetyl groups to bring back the desired qualities or solubilities in the product, and to do this without the use of expensive ingredients, such as acetic anhydrid.

We have found that the desired results may be obtained by heating the cellulose acetate, from which the acetyl groups have been split oil, with acetic acid as the sole acetylating agent. This reintroduces acetyl groups in a positive manner and yet does it at a rate which places the proportion introduced under accurate control. For example, cellulose acetate, which has been hydrolyzed until the original acetyl percentage has dropped to the point where the product is no longer adequately soluble in acetone, can be restored to acetonesolubility by boiling in substantially anhydrous acetic acid for a moderate length of time of about the order of magnitude of 24 hours. Moreover, it may be rendered also soluble in chloroform by restoring substantially most of the acetyl groups upon boiling in acetic acid for a time of the order of magnitude of .200 hours.

' groups with the same solubilities.

This same treatment will acetylate cellulose at the desired stage, thus very accurately acetates from which the acetyl groups have been even entirely removed. I stored to each molecule jof cellulose acetate. It is well known that different batches of It wlll be noted that the acetic acid is the cellulose acetate vary somewhat in theirsussole acetylating agent and we do not require ceptibilities toward difi'erent solvents, but the use of expensive acetic anhydrid, or even the specimen referred to in the following the use of a catalyst. table illustrates a common type. p When workmg at atmospheric pressure the acid is boiled and thus thereaction carried on at about 117 C. It proceeds at a t Chloroform Acetone warm ethyl ggr l alcoholuseful rate, however, even at somewhat lower temperatures, preferably above 100 .44.8 Solulgle. i nsotllublei bl nso}ug}e. L/IOI'QOVBT, hlgher temperatures may be 42. S at so u 9. use 11 e.

1, 3 3, Insolubleeniployed when superatmospheric pressure gargysol gl f l gl e- 1 M Y p l l me is employed. As a general proposition, we

3. e. eal sou e. or on a 35.2 ifs opig gengigslolgple. garfig soiugle. p lefer to keep the temperatures below 170 34.2 I so e. at ysou e. or ysou e.

328 3 3, sow, 1n older to avoid, molecular degradation 1-1 In olu l In l Soluble of the products, as evidenced by the br1ttleness of films formed from them. When su- The first column shows the percentage of peratmospheric pressure is employed, the acetyl group which is present in the cellulose time to roduce a given reacetylation is reacetate, as the hydrolysis with aclds produced. ut the process proceeds at a satisgresses. The second column shows how the factory rate when the acid is boiled at atgauging the amount of acetyl groups resolubility in chloroform diminishes and niospheric pressure, and this is the preferred disappears as the percentage of acetyl form.

groups drops. The third column shows how It is desirable to use substantially anhythe solubility in acetone grows to a 'maxldrous acid and to remove at least some of mum and then diminishes until it disapthe water formed during the reaction. pears. The fourth column shows the de- These conditions are satisfactorily met by velopment of solubility in warm ethyl alcousing a fractional reflux condenser, which 1101. This illustrates changes which may be will condense and return to the bath the brought about by ordinary methods of hyvapors of acetic acid Without condensing drolysis. It is apparent that carelessness the water vapor. or inadvertence may cause the hydrolysis Having thus described our invention, what to be prolonged to a point where the desired we claim as new and desire to secure by Let solubility, say in acetone, is impaired or even ters Patent is: lost altogether. l The process of at least partially re- Our process may then be used for restoracetylating cellulose acetate from which at ing desired solubility by restoring or replacleast part of the acetyl groups have been reing at least some of the acetyl groups. For moved until its solubility has thereby been example, a cellulose acetate, which was origiimpaired, which comprises heating said celnally soluble in chloroform, but not in ace lulose acetate with acetic acid as the sole tone, and contained nearly 45% of acetyl acetylating agent until further acetyl groups groups, was hydrolyzed by acids in the well are introduced to correct said impaired soluknown way until it contained only 31.2% bility, the temperature being kept below the of acetyl groups and had become entirely indecomposition point of the product. soluble in either acetone or chloroform. 2. The process of at least partially re- One part of this was boiled, at atmospheric acetylating cellulose acetate, the solubility of pressure, with 20 parts by weight of acetic which has been changed by removal of acetyl acid. After 2-l hours boiling the product groups by hydrolysis, which comprises heatcontained 37.3% of acetyl groups and was ing said cellulose acetate with acetic acid readily soluble in acetone and partially as the sole acylating agent to temperatures soluble in chloroform. This brought it to above 100 C. but below 170 C. until acetyl the stage where it was in its best commercial groups are introduced to replace those reform as regards acetone-solubility. A simimoved and restore the solubility which exlar process conducted for 48 hours, on the isted before said removal. same sample, brought about an acetyl con- 3. The process of correcting the solubility tent of 40.5% thus giving a fair solubility of cellulose acetate, formed by excessive hyin both chloroform and acetone. The procdrolysis of chloroform-soluble cellulose aceess was carried out on the same sample for tate until acetone-solubility is produced and 200 hours until it contained 42.3% of acetyl then impaired, which comprises heating said It will hydrolyzed acetate with acetic acid as the be apparent that this process proceeds at sole acetylatingagent between 100 C. and such a rate that samples can readily be with 170 C. until acetyl groups are introduced to drawn and tested and the action stopped correct said impaired acetone-solubility.

4. The. process of partially reacetylating remove water formed by the reaction, until cellulose acetate, the solubility of whlch has acetyl groups are introduced to replace those been changed by removal of acetyl groups removed and restore the solubility. 10

by h clrolysis, which comprises boiling said Signed at Rochester, New York, this 7th 5 cellu os'e acetate at atmospheric pressure day of January, 1927.

with substantially anhydrous acetic acid HANS T. CLARKE.

with fractional refluxing condensation, to CARL J. MALM. 

